Polymers their preparation and uses

ABSTRACT

A polymer for use in an optical device comprising a first, optionally substituted, repeat unit of formula  
                 
 
and a second, optionally substituted, repeat unit of formula  
                 
wherein each Ar and Ar′ is the same or different and comprises an optionally substituted aryl or heteroaryl group and optionally a third, optionally substituted, repeat unit in a molar ratio of no greater than 5%, the third repeat unit having a formula —Ar—N(Ar)—Ar— and having a single nitrogen atom in its backbone.

FIELD OF THE INVENTION

The invention relates to materials for optical devices, in particularorganic electroluminescent devices, and the control of their physicaland electronic properties.

BACKGROUND OF THE INVENTION

One class of opto-electrical devices is those using an organic materialfor light emission or detection. The basic structure of these devices isa light emissive organic layer, for instance a film of a poly(p-phenylenevinylene) (“PPV”) or polyfluorene, sandwiched between acathode for injecting negative charge carriers (electrons) and an anodefor injecting positive charge carriers (holes) into the organic layer.The electrons and holes combine in the organic layer generating photons.In WO 90/13148 the organic light-emissive material is a polymer. In U.S.Pat. No. 4,539,507 the organic light-emissive material is of the classknown as small molecule materials, such as (8-hydroxyquinoline)aluminium (“Alq3”). In a practical device one of the electrodes istransparent, to allow the photons to escape the device.

A typical organic light-emissive device (“OLED”) is fabricated on aglass or plastic substrate coated with a transparent first electrodesuch as indium-tin-oxide (“ITO”). A layer of a thin film of at least oneelectroluminescent organic material covers the first electrode. Finally,a cathode covers the layer of electroluminescent organic material. Thecathode is typically a metal or alloy and may comprise a single layer,such as aluminium, or a plurality of layers such as calcium andaluminium. Other layers can be added to the device, for example toimprove charge injection from the electrodes to the electroluminescentmaterial. For example, a hole injection layer such as poly(ethylenedioxythiophene)/polystyrene sulfonate (PEDOT-PSS) or polyaniline may beprovided between the anode and the electroluminescent material. When avoltage is applied between the electrodes from a power supply one of theelectrodes acts as a cathode and the other as an anode

For organic semiconductors important characteristics are the bindingenergies, measured with respect to the vacuum level of the electronicenergy levels, particularly the “highest occupied molecular orbital”(HOMO) and the “lowest unoccupied molecular orbital” (LUMO) level. Thesecan be estimated from measurements of photoemission and particularlymeasurements of the electrochemical potentials for oxidation andreduction. It is well understood in this field that such energies areaffected by a number of factors, such as the local environment near aninterface, and the point on the curve (peak) from which the value isdetermined. Accordingly, the use of such values is indicative ratherthan quantitative.

In operation, holes are injected into the device through the anode andelectrons are injected into the device through the cathode. The holesand electrons combine in the organic electroluminescent layer to form anexciton which then undergoes radiative decay to give light. One way ofimproving efficiency of devices is to provide hole and electrontransporting materials—for example, WO 99/48610 discloses blending ofhole transporting polymers, electron transporting polymers andelectroluminescent polymers. A 1:1 copolymer of dioctylfluorene andtriphenylamine is used as the hole transporting polymer in thisdocument.

A focus in the field of polymer OLEDs is the development of full colourdisplays for which red, green and blue emissive materials are required.One drawback with existing polymer OLED displays relevant to thisdevelopment is the relatively short lifetime of blue emissive materialsknown to date (by “lifetime” is meant the time for the brightness of theOLED to halve at constant current when operated under DC drive).

In one approach, the lifetime of the emissive material may be extendedby optimisation of the OLED architecture; for example lifetime of theblue material may in part be dependant on the cathode being used.However, the advantage of selecting a cathode that improves bluelifetime may be offset by disadvantageous effects of the cathode onperformance of red and green materials. For example, Synthetic Metals111-112 (2000), 125-128 discloses a full colour display wherein thecathode is LiF/Ca/Al. The present inventors have found that this cathodeis particularly efficacious with respect to the blue emissive materialbut which shows poor performance with respect to green and, especially,red emitters.

Another approach is development of novel blue electroluminescentmaterials. For example, WO 00/55927, which is a development of WO99/48160, discloses a blue electroluminescent polymer of formula (a):

wherein w+x+y=1,w≧0.5, 0≦x+y≦0.5and n≧2

In essence, the separate polymers disclosed in WO 99/48160 are combinedinto a single molecule. The F8 repeat unit is provided for the purposeof electron injection; the TFB unit is provided for the purpose of holetransport; and the PFB repeat unit is provided as the emissive unit.

WO 99/54385 and EP 1229063 disclose copolymers of fluorenes and amines.

It is an object of the present invention to provide a means forincreasing the lifetime of polymers for use in an optical device abovethat of prior art polymers. It is a further object of the invention toprovide a long-lived polymer for use in an optical device, particularlya long-lived blue electroluminescent material. It is a yet furtherobject of the invention to provide a means for increasing the thermalstability of prior art polymers.

SUMMARY OF THE INVENTION

The present inventors have surprisingly found that the lifetime of apolymer for use in an optical device, in particular anelectroluminescent polymer, may be increased by the incorporation ofrepeat units that increase the glass temperature (Tg) of the polymer. Inparticular, incorporation of 2,7-linked 9,9-diarylfluorene repeat unitsinto an electroluminescent polymer, particularly a blue emissiveelectroluminescent polymer, results in significant increase in thatpolymer's lifetime. Furthermore, the present inventors have found thatit is unnecessary to have separate hole transporting units and blueemissive units; it has been found that both functions may be performedby the PFB unit Surprisingly, the omission of TFB from the prior artpolymers described above is found to result in a significant improvementin lifetime.

Accordingly, in a first aspect the invention provides a polymer for usein an optical device comprising a first, optionally substituted, repeatunit of formula (I) and a second, optionally substituted, repeat unit offormula (II):

wherein each Ar and Ar′ is the same or different and comprises anoptionally substituted aryl or heteroaryl group and, optionally, athird, optionally substituted, repeat unit in a molar ratio of nogreater than 5%, the third repeat unit having a formula —Ar—N(Ar)—Ar—and having a single nitrogen atom in its backbone.

Preferably, each Ar is phenyl. More preferably, the second repeat unitcomprises a repeat unit of formula (III):

wherein each R′ is independently selected from hydrogen or asolubilising group.

Preferably, the third repeat unit is absent. Preferably, the polymercomprises no repeat units comprising nitrogen atoms in the repeat unitbackbone other than the repeat unit of formula (II).

Accordingly, in a first aspect the invention provides a polymer for usein an optical device comprising an optionally substituted repeat unit offormula (I) and an optionally substituted repeat unit of formula (II):

wherein each Ar and Ar′ is the same or different and comprises anoptionally substituted aryl or heteroaryl group and optionally a third,optionally substituted, repeat unit in a molar ratio of no greater than5%, the third, repeat having formula —Ar—N(Ar)—Ar— and having a singlenitrogen atom in its backbone.

Preferably, each Ar is independently selected from the group comprisingan optionally substituted residue of formula (IV):

wherein n=1, 2 or 3 and R is a solubilising group or hydrogen. Preferredsolubilising groups R are selected from optionally substituted alkyl oralkoxy, preferably butyl.

Preferably, each R′ is optionally substituted alkyl or alkoxy, morepreferably butyl, most preferably n-butyl.

Preferably, the polymer according to the first aspect of the inventioncomprises less than 50 mol %, more preferably 10-40 mol %, of repeatunits of formula (I).

The present inventors have surprisingly found that reducing the aminecontent within a polymer results in an increase in the efficiency ofthat material in certain device architectures, in particular deviceswith cathodes having a relatively high workfunction resulting inrelatively poor electron injecting ability such as barium (wf=2.7 eV),strontium (wf=2.59 eV) or calcium (wf=2.87 eV) (source: J. Appl. Phys.48(11) 1997, 4730.) Accordingly, in one preferred embodiment the polymeraccording to the first aspect of the invention comprises less than 30mol %, more preferably less than 10 mol %, of repeat units of formula(II).

Preferably, the polymer according to the first aspect of the inventioncomprises a further repeat unit selected from optionally substituted9,9-dialkyl- or 9,9-dialkoxy-2,7-fluorennyl, more preferably9,9-di(n-octyl)fluorene.

Preferably, the polymer according to the first aspect of the inventionis an electroluminescent polymer, more preferably a polymer capable ofemitting light in the wavelength range 400-500 nm, most preferably430-500 nm

In a second aspect, the invention provides an electroluminescent devicecomprising a first electrode for injection of positive charge carriers,a second electrode for injection of negative charge carriers and anelectroluminescent region located between the first and second electrodecomprising a polymer according to the first aspect of the invention.Preferably, the polymer according to the first aspect of the inventionis the only semiconducting polymer in the electroluminescent region.

By “electroluminescent region” is meant that layer of anelectroluminescent device comprising a polymer comprising a repeat unitof formula (II) from which electroluminescence is obtained. For theavoidance of doubt, it will be appreciated that, where present, a holeinjection material (such as PEDOT-PSS or polyaniline), a holetransporting layer or an electron transporting layer separate from theelectroluminescent layer do not constitute a part of theelectroluminescent region.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in further detail, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 shows a prior art electroluminescent device

FIG. 2 shows a plot of luminance vs. time for a blue electroluminescentdevice according to the invention

DETAILED DESCRIPTION OF THE INVENTION

Polymers according to the invention are preferably copolymers comprisingan arylene co-repeat unit Ar such as a fluorene, particularly 2,7-linked9,9 dialkyl fluorene or 2,7-linked 9,9 diaryl fluorene; a spirofluorenesuch as 2,7-linked 9,9-spirofluorene; an indenofluorene such as a2,7-linked indenofluorene; or a phenyl such as alkyl or alkoxysubstituted 1,4-phenylene. Each of these groups may be substituted.

Further suitable Ar groups are known in this art, for example asdisclosed in WO 00/55927 and WO 00/46321, the contents of which areincorporated herein by reference.

A polymer according to the present invention may comprise a homopolymer,copolymer, terpolymer or higher order polymer.

A copolymer, terpolymer or higher order polymer according to the presentinvention includes regular alternating, random and block polymers wherethe percentage of each monomer used to prepare the polymer may vary.

For ease of processing, it is preferred that the polymer is soluble.Substituents such as C₁₋₁₀ alkyl or C₁₋₁₀ alkoxy may usefully beselected to confer on the polymer solubility in a particular solventsystem. Typical solvents include mono- or poly-alkylated benzenes suchas toluene and xylene or THF.

Two polymerisation techniques that are particularly amenable topreparation of conjugated polymers from aromatic monomers are Suzukipolymerisarion as disclosed in, for example, WO 00/53656 and Yamamotopolymerisation as disclosed in, for example, “Macromolecules”, 31,1099-1103 (1998). Suzuki polymerisation entails the coupling of halideand boron derivative functional groups; Yamamoto polymerisation entailsthe coupling of halide functional groups. Accordingly, it is preferredthat each monomer is provided with two reactive functional groups Pwherein each P is independently selected from the group consisting of(a) boron derivative functional groups selected from boronic acidgroups, boronic ester groups and borane groups and (b) halide functionalgroups.

With reference to FIG. 1, the standard architecture of an optical deviceaccording to the invention, in particular an electroluminescent device,comprises a transparent glass or plastic substrate 1, an anode of indiumtin oxide 2 and a cathode 4. The polymer according to the invention islocated in layer 3 between anode 2 and cathode 4. Layer 4 may comprisethe polymer according to the invention alone or a plurality of polymers.Where a plurality of polymers are deposited, they may comprise a blendof at least two of a hole transporting polymer, an electron transportingpolymer and, where the device is a PLED, an emissive polymer asdisclosed in WO 99/48160. Alternatively, layer 3 may be formed from asingle polymer that comprises regions selected from two or more of holetransporting regions, electron transporting regions and emissive regionsas disclosed in, for example, WO 00/55927 and U.S. Pat. No. 6,353,083.Each of the functions of hole transport, electron transport and emissionmay be provided by separate polymers or separate regions of a singlepolymer. Alternatively, more than one function may be performed by asingle region or polymer. In particular, a single polymer or region maybe capable of both charge transport and emission. Each region maycomprise a single repeat unit, e.g. a triarylamine repeat unit may be ahole transporting region. Alternatively, each region may be a chain ofrepeat units, such as a chain of polyfluorene units as an electrontransporting region. The different regions within such a polymer may beprovided along the polymer backbone, as per U.S. Pat. No. 6,353,083, oras groups pendant from the polymer backbone as per WO 01/62869.

In addition to layer 3, a separate hole transporting layer and/or anelectron transporting layer may be provided.

Although not essential, a layer of organic hole injection material (notshown) between the anode 2 and the polymer layer 3 is desirable becauseit assists hole injection from the anode into the layer or layers ofsemiconducting polymer. Examples of organic hole injection materialsinclude poly(ethylene dioxythiophene) (PEDT/PSS) as disclosed in EP0901176 and EP 0947123, or polyaniline as disclosed in U.S. Pat. No.5,723,873 and U.S. Pat. No. 5,798,170.

Cathode 4 is selected from materials that have a workfunction allowinginjection of electrons into the electroluminescent layer. Other factorsinfluence the selection of the cathode such as the possibility ofadverse interactions between the cathode and the electroluminescentmaterial. The cathode may consist of a single material such as a layerof aluminium. Alternatively, it may comprise a plurality of metals, forexample a bilayer of calcium and aluminium as disclosed in WO 98/10621,elemental barium disclosed in WO 98/57381, Appl. Phys. Lett. 2002,81(4), 634 and WO 02/84759 or a thin layer of dielectric material toassist electron injection, for example lithium fluoride disclosed in WO00/48258 or barium fluoride, disclosed in Appl. Phys. Lett. 2001, 79(5),2001.

A typical electroluminescent device comprises an anode having aworkfunction of 4.8 eV. Accordingly, the HOMO level of the holetransporting region is preferably around 4.8-5.5 eV. Similarly, thecathode of a typical device will have a workfunction of around 3 eV.Accordingly, the LUMO level of the electron transporting region ispreferably around 3-3.5 eV.

Electroluminescent devices may be monochrome devices or full colourdevices (i.e. formed from red, green and blue electroluminescentmaterials).

EXAMPLES Monomer Examples

Monomers according to the invention were prepared in accordance with thescheme below:

Monomer Example M1: 2,7-dibromo-9,9-diphenylfluorene

In a 3 L flange flask fluorenone (100.006 g, 0.555 mol), phosphoruspentoxide (110.148 g, 0.776 mol) and trimethylphosphate (1200 mL) weremixed. Under mechanical stirring, a solution of bromine (63 mL, 1.23mol) in trimethylphosphate (200 mL) was quickly added. This clearsolution was then heated for 22 hours at 120° C. The mixture was allowedto cool to room temperature, then poured into 3 L of water. When sodiumthiosulfate was added (50.045 g) the mixture turned yellow. Stirring wasmaintained for 1 hour then the yellow solid was filtered. This solid washeated in methanol to remove the mono-brominated compound and gave176.183 g (98% pure by HPLC, 94% yield). ¹H NMR (CDCl₃) 7.73 (2H, d, J2.0), 7.61 (2H, dd, J 7.6, 2.0), 7.36 (2H, d, J 8.0); ¹³C NMR (CDCl₃)142.3, 137.5, 135.3, 127.9, 123.3, 121.8, 109.8.

In a 2 L flange flask 2,7-dibromofluorenone (120.526 g, 0.356 mol),potassium hydroxide (finely powdered flakes, 168.327 g, 3.000 mol) andtoluene (600 mL) were placed. This mixture was heated at 120° C. forfour hours then left to cool to room temperature. Water was added todissolve the solid (˜2 L) under vigorous stirring. The greenish aqueouslayer was removed and the yellow toluene layer was washed twice withwater. The combined aqueous layers were acidified with concentratedhydrochloric acid then the precipitated solid was filtered, dried thenrecrystallised from toluene to give 100.547 g of off white crystals (79%yield); ¹H NMR ((CD₃)₂CO) 8.00 (1H, d, J 2.0), 7.77 (1H, dd, J 8.0,2.4), 7.57 (2H, d, J 8.0), 7.34 (1H, d, J 8.4), 7.29 (2H, d, J 8.8) ;¹³C NMR ((CD₃)₂CO) 167.1, 140.4, 139.8, 134.2, 133.5, 132.8, 132.7,131.2, 130.6, 121.4, 121.1.

4,4-dibromo-2-carboxylic acid biphenyl (171.14 g, 0.481 mol) wassuspended in methanol (700 mL) and sulfuric acid (15 mL) then heated at80 ° C. for 21 hours. The solvent was removed and the oil was dissolvedin ethyl acetate. This solution was washed with 2N sodium hydroxide,water, saturated sodium chloride, dried over magnesium sulfate, filteredand evaporated to give an orange oil. This oil was treated with hotmethanol, on cooling the ester precipitated out and was filtered. Themother liquor was evaporated and the solid recrystallised givingadditional product The ester was 100% pure by GCMS, a yield of 123.27 g(69%) was obtained; ¹H NMR (CDCl₃) 7.99 (1H, d, J 2.0), 7.64 (1H, dd, J8.0, 1.6), 7.51 (2H, d, J 8.4), 7.19 (1H, d, J 8.8), 7.13 (2H, d, J 8.8), 3.67 (3H, s); ¹³C NMR (CDCl₃) 167.1, 140.3, 139.1, 134.4, 132.9,132.1, 132.0, 131.3, 129.8, 121.9, 121.5, 52.3; GCMS: M⁺=370

4,4-dibromo-2-methyl ester-biphenyl (24.114 g, 65.1 mmol) was dissolvedin dry diethyl ether (120 mL) and the solution was cooled to −60° C. byusing an isopropanol/dry ice bath. Phenyl lithium (1.8M solution incyclohexane-ether, 91 mL) was then added dropwise. The mixture wasstirred and let to warm to room temperature. The reaction was completeafter four hours. Water was added (70 mL) then the aqueous layer washedonce with diethyl ether. Combined organic phases were washed with sodiumchloride, dried over magnesium sulfate, filtered and evaporated to givea yellow powder. Recrystallisation from isopropanol afforded 19 g ofwhite solid (59% yield); GC-MS (m/z, relative intensity %) 494 (M⁺,100); ¹H NMR (CDCl₃) 7.43 (1H, dd, J 8.4, 2.4), 7.28 (6H, m), 7.23 (2H,d, J 8.0), 7.11 (4H, m), 6.99 (1H, d, J 2.4), 6.94 (1H, d, J 8.4), 6.61(2H, d, J 8.4); ¹³C NMR (CDCl₃) 147.5, 146.7, 140.3, 139.3, 134.0,133.0, 131.2, 131.1, 130.3, 128.2, 128.1, 127.8, 121.8, 121.3, 83.2.

The alcohol (69.169 g, 140 mmol) and glacial acetic acid (450 ml) werestirred and heated to reflux, then concentrated hydrochloric acid (0.5ml) was added dropwise. When the addition was completed the mixture washeated for one hour and then cooled. The reaction mixture was pouredinto water (500 ml), after which the solid was filtered off. The whitesolid was recrystallised from n-butyl acetate three times to give 20.03g of desired product (99.59% by HPLC, 30% yield).

¹H NMR (CDCl₃), δ/ppm: 7.58 (2H, d, J 7.6), 7.49 (2H, d, 1.2), 7.48 (2H,dd, 1.6), 7.25 (6H, m), 7.14 (4H, m).

¹³C NMR (CDCl₃), δ/ppm: 153.2, 144.6, 138.3, 131.1, 129.6, 128.7, 128.2,127.4, 122.0, 121.7, 65.8.

Monomer Examples M2-M4

Monomers with Ar groups as detailed in the table below were prepared inaccordance with the scheme and general experimental process outlinedabove. Aryllithium compounds corresponding to Ar groups shown in thetable were prepared from the corresponding aryl bromide. Monomer Yieldof example no. Ar monomer M2

90% M3

24% M4

22%

Polymer Example P1

A blue electroluminescent polymer according to the invention wasprepared in accordance with the process of WO 00/53656 by reaction of9,9-di-n-octylfluorene-2,7-di (ethylenylboronate) (0.5 equivalents),2,7-dibromo-9,9-diphenylfluorene (0.35 equivalents) andN,N′-di(4-bromophenyl)-N,N′-di(4-n-butylphenyl)-1,4-diaminobenzene (0.15equivalents) to give polymer P1:

Device Example

Onto indium tin oxide supported on a glass substrate {available fromApplied Films, Colorado, USA) was deposited a layer of PEDT/PSS,available from Bayer® as Baytron P® by spin coating. A layer of polymerP1 was deposited over the PEDT/PSS layer by spin-coating from xylenesolution. Onto the polymer P1 was deposited a cathode consisting of afirst layer of lithium fluoride, a second layer of calcium and a third,capping layer of aluminium.

For the purpose of comparison with P1, an identical device was preparedexcept that the electroluminescent polymer comprised 10% TFB repeatunits (full composition: 10% TFB; 50% F8; 30% PFB; 10% PFB—disclosed inWO 02/92723).

As can be seen from FIG. 1, removal of TFB from the polymer results in asignificant improvement in lifetime.

Although the present invention has been described in terms of specificexemplary embodiments, it will be appreciated that variousmodifications, alterations and/or combinations of features disclosedherein will be apparent to those skilled in the art without departingfrom the spirit and scope of the invention as set forth in the followingclaims.

1. A polymer for use in an optical device comprising a first, optionallysubstituted, repeat unit of formula (I) and a second, optionallysubstituted, repeat unit of formula (II):

wherein each Ar and Ar′ is the same or different and comprises anoptionally substituted aryl or heteroaryl group and optionally a third,optionally substituted, repeat unit in a molar ratio of no greater than5%, the third repeat unit having a fonnula —Ar—N(Ar)—Ar— and having asingle nitrogen atom in its backbone.
 2. A polymer according to claim 1wherein the second repeat unit comprises a repeat unit of formula (III):

wherein each R′ is independently selected from the group consisting ofhydrogen and solubilizing groups.
 3. A polymer according to claim 1wherein the third repeat unit is absent.
 4. A polymer according to claim1 wherein each Ar is independently selected from the group consisting ofoptionally substituted residues of formula (IV):

wherein n=1, 2 or 3 and R is a solubilizing group or hydrogen.
 5. Apolymer according to claim 4 wherein R is selected from the groupconsisting of optionally substituted alkyl and alkoxy groups.
 6. Apolymer according to claim 4 wherein R is hydrogen or butyl.
 7. Apolymer according to claim 1 wherein each R′ is optionally substitutedalkyl or alkoxy.
 8. A polymer according to claim 7 wherein R′ isn-butyl.
 9. A polymer according to claim 1 comprising less than 50 mol %of repeat units of formula (I).
 10. A polymer according to claim 1comprising less than 30 mol % of repeat units of formula (II).
 11. Apolymer according to claim 1 comprising less than 10 mol % of repeatunits of formula (II).
 12. A polymer according to claim 1 comprising afurther repeat unit selected from the group consisting of optionallysubstituted 9,9-dialkyl- and 9,9-dialkoxy-2,7-fluorenyl groups.
 13. Apolymer according to claim 12 wherein the further repeat unit is9,9-di(n-octyl)fluorene.
 14. A polymer according to claim 9 comprising10-40 mol % of repeat units of formula (I).
 15. A polymer according toclaim 1 capable of emitting light in the wavelength range 400-500 nm.16. An optical device comprising a first electrode for injection ofpositive charge carriers, a second electrode for injection of negativecharge carriers and a layer located between the first and secondelectrode comprising a polymer as defined in claim
 1. 17. An opticaldevice according to claim 16 that is an electroluminescent device. 18.An optical device according to claim 17 wherein said polymer is the onlysemiconducting polymer in the electroluminescent region.
 19. A polymeraccording to claim 1 capable of emitting light in the wavelength range430-500 nm.